The present invention generally relates to apparatus for wet grinding and, more particularly, to the grinding of particles in a slurry solution down to particles having a size on the order of several microns or less.
Generally, size reduction of particles in a slurry is accomplished in multistage processes. That is, large particles of grain such as whole grain, corn, rice and the like, or other solids are initially broken down in size by conventional milling apparatus such as roll crushers, hammer mills, shredders and other similar devices depending on the product being ground. As one example, hammer mills may be suitable for use in dry grinding processes, wet grinding processes or both and may include a rotating cylinder or drum with attached "hammers" which crush particles against a stator screen. Hammer mills generally work under the principle of forcing particles through the holes in the stator screen to produce particles of a size commensurate with the size of the screen holes. Hammer mills are used in the distilling industry to dry grind grain which is later slurried with water in a separate tank to prepare the grain for fermentation. Disadvantages of hammer mills include the possibility of explosions resulting from the production of large amounts of dust, high maintenance costs associated with regular replacement of stator screens, and loss of product and damage to product from heat produced during the grinding process.
U.S. Pat. No. 4,813,617 ('617) issued on Mar. 21, 1989 to Knox, Jr. et al., which names the present inventor as a co-inventor thereof, addresses the problem of obtaining both maximum grinding efficiency and maximum throughput volume in a wet grinding machine. The '617 patent provides apparatus for very efficiently grinding large particles such as corn and the like down to smaller particles, for example, on the order of 1/8" in diameter, on a continuous high throughput volume basis. The '617 patent successfully accomplishes this objective by combining both large and small slots in a stationary stator and using a rotating bladed rotor disposed within the stator. Larger particles are reduced in size through shearing action between the blades of the rotor and the edges of the large slots in the stator and smaller particles are reduced in size through shearing action between the rotor blades and the small slots in the stator. Large particles are transferred out of the stator through the large slots and small particles are transferred out of the stator through both the large and small slots. The apparatus disclosed in the '617 patent presents a significant improvement over past grinding methods in terms of the size reduction and throughput volume potential of a single step grinding apparatus capable of reducing relatively large particles down to particles having an average diameter, for example, of 1/8".
Regarding apparatus and methods for reducing particles from a size on the order of 1/8" to a size of several microns or less, ball mills, hammer mills and homogenizers have been used in the past. Apparatus of this type have several undesirable features and cost implications. First, in order to obtain smaller and smaller particle sizes the holes in the stator screen or, for example, the balls or beads of a ball mill must be smaller to obtain smaller particle sizes. As the screen holes, balls or beads get smaller so to does the throughput volume of the grinding apparatus using these grinding or size reduction means. Thus, past fine grinding methods produce very low volumes of finely ground product.
Also, the costs associated with the manufacture, operation and maintenance of these machines is very high. For example, the costs associated with manufacturing minute openings in the screens used in a hammer mill are high especially when considering that the screens must be replaced constantly. The costs of manufacturing and maintaining a typical homogenizer are high due to the costs of the high pressure pumps, high powered motors and many other precision components.
Other problems have arisen using past methods to produce particle sizes on the order of several microns or less such as the undesirably long milling times, which may stretch up to 30 hours and which add to the costs of using ball mills, hammer mills and homogenizers. In the case of ball mills, due to the long milling time involved, these mills must be surrounded by cooling jackets which further add to their cost and complexity.
Accordingly, there is a need in the art for apparatus and methods for reducing the size of particles from sizes easily produced by apparatus such as that shown in the '617 patent, down to sizes on the order of several microns or less in a fast and efficient manner by producing continuous high throughput volumes of dispersions and emulsifications containing such particles.
It has therefore been one objective of the invention to provide a wet fine grinder capable of continuously grinding particles contained in a slurry solution without clogging and without significant wear on the size reducing or grinding components of the apparatus.
It has been another objective of the invention to produce high throughput volumes of particles of a size on the order of several microns or less quickly and efficiently on a continuous in-line basis as opposed to a single batch basis.
It has been still another objective of the invention to significantly reduce the amount of time and number of grinding steps necessary to reduce large amounts of slurry solution containing relatively large particles into a slurry solution containing particles of several microns or less in size.
It has been yet another objective of the invention to provide apparatus for grinding particles in a batch of slurry contained in a tank as well as apparatus for grinding particles contained in slurry solution traveling in a fluid line and continuously recirculating slurry solution through each grinding apparatus.
It has been still a further objective of the invention to use the slurry solution itself as a lubricant and a coolant for the grinding components of the apparatus to substantially reduce wear on grinding components of the apparatus.